The present invention relates to a method and a device for recognizing defects in a triggering system of a controlled series compensator.
In series compensation systems, capacitors are usually employed in the wiring run to reduce the current-dependent voltage drop across the line and the transmission angle, in steps. These capacitors are capacitor banks, which are switched on and off in series, as a whole or in several capacitor sections (segments). The capacitor is switched on and off by opening or closing a parallel circuit-breaker. In case of a short-circuit on the network, a parallel arrester, a triggerable spark gap, and/or a parallel circuit-breaker guarantee protection for the capacitor.
Furthermore, a series compensation system is known, in which by means of an inductor connected in parallel with the capacitor, the total impedance of this series capacitor, similarly to the case of a TCR (thyristor controlled reactor) in the static compensator, is steplessly controlled with a current-converter valve to high-voltage potential through appropriate triggering. A series compensator controlled in this manner is known as an ASC (Advanced Series Compensation). A series compensator controlled in this manner allows the dynamic response of the series compensation to be improved, and the total impedance is able to be controlled automatically within a certain range, whereby the impedance can be changed from capacitive to inductive impedance.
These types of series capacitors are introduced in the essay, Automatically Controlled Parallel- and Series Compensation [Geregelte Parallel- und Reihenkompensation], published in the German periodical, "Elektrie", Volume 45, 1991, March pp. 88 through 90. In addition, the International Patent WO 87/04538 describes a series capacitor, which is controlled in such a manner and is integrated in a transmission line.
The impedance of the series compensator is adjusted by influencing the firing angle of the current-converter valve, which is electrically connected in parallel via a coil to the series capacitor of the series compensator. The current-converter valve consists of two antiparallel connected valves, which are each comprised of several series-connected valves. The impedance to be adjusted is predetermined by primary coordinating (conduction current control, load voltage control, oscillation damping). In an arrangement of the series compensator in a three-phase voltage system, the impedance to be adjusted is of the same magnitude in the three phases. The firing angle .alpha., which applies to the alternating component of the capacitor voltage, has a non-linear relation to the impedance.
When there are defects in the triggering system of the valves, it can be that the impedance of the controlled series compensator does not equal the setpoint impedance required by a closed-loop control or by an open-loop control. This state of the controlled series compensator results in disadvantages for the network (asymmetry, unequal loading of the lines, different degree of compensation). It can happen that the advantages of the controlled series compensator are lost, or that these advantages turn into disadvantages.
Therefore, this fault condition in the controlled series compensator must be able to be detected by the series compensator itself, since this fault condition can only be detected with the existing devices in the network after a delay, or it cannot be detected at all. Thus, the controlled series compensator requires a self-monitoring system.
Defects in the triggering system often result in a changed firing performance for the current-converter valve, i.e., the current-converter valve is brought into circuit after a delay or is not brought into circuit at all. Of the possible defects, one special case would be the failure of the valve electronics at a single valve location. The valve can only be brought into circuit by operating the overvoltage damping diode, also known as a break-over diode (BOD), which serves as overvoltage protection. The operating response voltage of an overvoltage damping diode is only slightly less than the maximum positive blocking voltage of the corresponding valve. In dependence upon the level of the capacitor voltage of the series capacitor of the controlled series compensator and upon the firing angle, the operating voltage of the overvoltage damping diode is reached and the valve is brought into circuit with delay. If the operating voltage of the overvoltage damping diode is not reached at other system working points, the result is a firing failure, which means that the controlled series compensator only still has a restricted control range at its disposal.
Apart from the mentioned case, all defects in the triggering system result in a failure of the valve firing.
Faults in the closed-loop control system and in the triggering system must be detected by devices of the controlled series compensator and, in this respect, this series compensator must be brought into a state in which disadvantages for the network operation are avoided.